Add the vacation message right in your theme using the inline block.
In the Shopify admin, go to Online Store → Themes.
Find your theme and click Customize.
Use the top dropdown to pick the product template you want.
Choose where the message should show, then click + Add block or + Add section.
In Apps, select Inline message.
Click Save.
If Inline is turned on for a rule, the message will appear in the spot you added.